A surface acoustic wave filter is used in a TV set for filtering a wave band of a TV signal of intermediate frequency so as to achieve easy control of the wave band and thus reduce the number of parts in a TV set.
A different wave band frequency is used for each channel for transmitting a TV signal. The TV signal is picked up by an antenna and passes through a filter circuit so as to be conveyed as a picture signal and a sound signal which are introduced into the TV receiver. The TV receiver comprises one common filter for all channels instead of providing one specific filter for each channel. This common filter is used for filtering a wave band of intermediate frequency which is lower than the frequency of the wave band for each channel. The wave band for each channel is shifted down to the common wave band of intermediate frequency at the time of changing the channel, so that a picture signal and a sound signal of each channel can be picked up through the common filter. A surface acoustic wave filter is used as the common filter.
A surface acoustic wave filter of the prior art with a wave band of intermediate frequency used in a TV receiver has a characteristic frequency which allows it to pass chroma carrier waves (c.c) of 55.17 MHZ, picture carrier waves (p.c) of 58.75 MHZ and sound carrier waves (s.c) of 54.25 MHZ for TV broadcasting in Japan, as shown in FIG. 1.
An example of such a surface acoustic wave filter is illustrated in FIG. 2. The surface acoustic wave filter comprises an input transducer 3 and an output transducer 4 formed on a substrate 1 of piezoelectric material. A multistrip coupler 2 comprising a plurality of conductive strip patterns is disposed between the input transducer 3 and the ouput transducer 4 so as to shift the wave track in order to improve the filtering ability.
Transfer efficiency of the wave from the track of input transducer 3 to the track of output transducer 4 is determined by the number of the parallel conductive strip patterns and the distance between any two adjacent patterns on the piezoelectric substrate 1. By disposing the multistrip coupler, an output transducer 4 is necesserily placed on the shifted position which is set off from the position facing the input transducer 3. Therefore, unnecessary bulk waves which advance straight within the substrate 1 from the input transducer 3 are not introduced into the output transducer 4.
Each of the transducers 3, 4 comprises a pair of comb shaped electrodes formed on the piezoelectric substrate 1 and interdigitated with each other. The characteristic of the filter depends upon the length of each tooth of the electrodes and the superposed length of adjacent teeth of the facing comb shaped electrodes. Therefore, by appropriately forming the comb, the desired characteristic of the filter corresponding to the picture and sound carriers can be obtained. The above mentioned method of obtaining the picture carrier and the sound carrier from the same wave signal of intermediate frequency is called an inter carrier system. In such an inter carrier system using the surface acoustic wave filter of the prior art, the sound signal and the picture signal are apt to be modulated and deformed, since the sound carrier, the chroma carrier and the picture carrier pass through one and the same filter so as to be amplified and detected.
Another method of obtaining the sound signal and the picture signal is called a split carrier method. A split carrier method using two completely separate filters requires a complicated filter circuit increasing the cost. Therefore, in another split carrier method, in order to obviate the above drawbacks, the sound signal and the picture signal are separated while passing through one common circuit.
An example of surface acoustic wave filter of the split carrier method of the prior art is illustrated in FIG. 3. This surface acoustic wave filter comprises a multistrip coupler 2 which changes one half of input wave power to another track and is formed on a substrate of piezoelectric material, an input transducer 3, formed on one side of the multistrip coupler 2 and two output transducers 4, 4' formed on the other side of the multistrip coupler 2. The input transducer 3 has wide band response corresponding to both transducers 4 and 4'. A signal from the input transducers 3 is transmitted to each of the output transducers 4, 4' through the multistrip coupler 2, as shown by the large outline arrows in FIG. 4. The picture signal and the sound signal can be separated by passing the input signal through the output transducers 4, 4', each of which transducers has a different characteristic. In such an arrangement, modulation of the picture signal and the sound signal can be avoided, the number of parts can be reduced and easy control of the signal can be achieved. However, in this arrangement, unnecessary waves which advance within the substrate 1 from the input transducer 3 enter the output transducers 4, 4' as shown by the dotted arrows in FIG. 4. Thus, as shown in FIG. 7, unnecessary waves 6, 6' appear on both sides of the desired wave 5 which corresponds to the characteristic of the filter. Also, adverse ripples 6" appear in the passing range of frequency of the filter as shown in FIG. 7.
These unnecessary waves 6, 6', 6" appear due to the bulk waves which advance straight within the substrate 1 and leak of acoustic wave during passing through the multistrip coupler from the input transducer 3 to the output transducer 4 or 4'. Also, a problem of transmission loss due to the insertion of the multistrip coupler arises in addition to the above disadvantages due to the unnecessary waves 6, 6' and 6".
Another example of surface acoustic wave filter of the split carrier type of the prior art is illustrated in FIG. 5. This surface acoustic wave filter comprises a multistrip coupler 2, two input transducers 3, 3' formed on one side of the multistrip coupler 2 and two output transducers 4, 4' formed on the other side of the multistrip coupler 2. The two input transducers 3, 3' are electrically connected in series with each other. A signal from each of the input transducers 3, 3' is transmitted to each of the output transducers 4, 4' through the multistrip coupler 2, as shown by the large outline arrows in FIG. 6. The picture signal and the sound signal can be separated by passing the input signals through the output transducers 4, 4', each of which transducers has a different characteristic. However, in such an arrangement, unnecessary waves 6, 6' and 6" similarly appear, as shown in FIG. 7, in the same manner as the former example.
The present invention was made in order to obviate these drawbacks of the prior art.